Characterization of a peculiar Einstein Probe transient EP240408a: an exotic gamma-ray burst or an abnormal jetted tidal disruption event?

Abstract

We present the results of our multi-wavelength (X-ray to radio) follow-up campaign of the Einstein Probe transient EP240408a. The initial 10 s trigger displayed bright soft X-ray (0.5-4 keV) radiation with peak luminosity Lₓ≳10⁴⁹ (10⁵⁰) erg s⁻¹ for an assumed redshift z>0.5 (2.0). The Neil Gehrels Swift Observatory and Neutron star Interior Composition ExploreR discovered a fading X-ray counterpart lasting for ∼5 d (observer frame), which showed a long-lived (~4 d) plateau-like emission (t⁻⁰.⁵) before a sharp powerlaw decline (t⁻⁷). The plateau emission was in excess of Lₓ≳10⁴⁶ (10⁴⁷) erg s⁻¹ at z>0.5 (2.0). Deep optical and radio observations resulted in non-detections of the transient. Our observations with Gemini South revealed a faint potential host galaxy (r≈24 AB mag) near the edge of the X-ray localization. The faint candidate host, and lack of other potential hosts (r≳26 AB mag; J≳23 AB mag), implies a higher redshift origin (z>0.5), which produces extreme X-ray properties that are inconsistent with many known extragalactic transient classes. In particular, the lack of a bright gamma-ray counterpart, with the isotropic-equivalent energy (10−10,000 keV) constrained by GECam and Konus-Wind to Eγ,iso≲4×10⁵¹ (6×10⁵²) erg at z>0.5 (2.0), conflicts with known gamma-ray bursts (GRBs) of similar X-ray luminosities. We therefore favor a jetted tidal disruption event (TDE) as the progenitor of EP240408a at z>1.0, possibly caused by the disruption of a white dwarf by an intermediate mass black hole. The alternative is that EP240408a may represent a new, previously unknown class of transient.